Mechanisms underlying direction-selectivity were studied in V1 of alert fixating macaque monkeys. Some direction-selective cells showed delayed asymmetric inhibition, some showed as shifting excitatory time-course across the receptive field, and some showed both. Both the direction of the spatial offset of the inhibition and the direction of the shift in excitatory response time-course correlated with the cells' preferred directionality. Part of the mechanism underlying a shifting response time-course may be delayed asymmetric inhibition. The data suggest that asymmetric inhibition is the major determinant for directionality in these cells, though both mechanisms could contribute. Based on this physiology, a simple, single-cell model is proposed, consistent with the known anatomy of some direction-selective cells in layer 6 (Meynert cells).